Class D amplifiers differ from conventional power amplifiers, such as Class A and Class B power amplifiers, in various respects. Conventional power amplifiers have an output voltage or current that is proportionally larger than its input. The output devices operate in the linear region, where the device is partially “on”. As a result, much of the power that is supplied to the amplifier converts to heat and is not efficiently utilized. Larger power transformers are needed as well as larger heat sinks, which remove heat from the output devices.
Unlike the partially “on”power amplifiers, Class D amplifiers operate such that the output devices are either on or off. Because the devices are on or off, the devices convert less power to heat. As a result, a smaller heat sink and power transformer can be used. However, there are some limitations associated with Class D amplifiers.
Due to power supply rejection, Class D amplifiers may suffer from reduced dynamic range and operate non-linearly. The power supply rejection ratio (“PSRR”) is the ratio of the change in output voltage of an amplifier to the change in power supply voltage, and can be expressed as 20*log (Voltage output change/Voltage supply change). In linear power supplies with a power transformer, as more current is consumed by the amplifier, the voltage provided by the power transformer is reduced. Class D amplifiers typically have a 0 dB PSRR. Any change in the power supply voltage is reflected directly at the output of the amplifier. If the power supply voltage is reduced by −6 dB for example, then the output signal will be reduced by −6 dB as well, reducing the dynamic range and causing non-linear operation. Dynamic range is the audio range from the lowest to the highest detectable volume signal output by the amplifier. If the reduction in the power rails reduces the highest level of amplifier output and maintains the level of the lowest signal, then the dynamic range is effectively reduced. As a result, the listener does not hear the louder passages as loud as intended, while the softer passages are produced as intended.
Audio power amplifiers are typically designed to drive loudspeakers. In subwoofer applications, it may be desirable to reduce the dynamic range under very controlled conditions using a compressor or limiter, which are types of automatic gain control circuits. A compressor changes the gain of an amplifier based on signal level. For example, an input of +6 dB into a compressor may result in an output of +3 dB when the signal is above a predetermined threshold. A limiter does not change the output with an increase of input signal, when the signal is above a set threshold. For example, an input of +6 dB into a limiter may result in no increase of signal on the output. Limiters and compressors can be expensive to implement. Most low cost compressor or limiter designs introduce harmonic distortion into the signal.
A conventional Class D audio amplifier as described in Motorola application note AN1042, “High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs” by Donald E. Pauly, which is hereby incorporated by reference. The design for power supply correction implements costly transformers. Additionally, the Audio Engineering Society preprint numbers 4446 and 4673 discuss error correction using feedback techniques and patented feed forward correction. However, feedback alone does not give enough correction for power supply errors.
Pulse Width Modulation is a well-known technique for synthesizing a train of pulses in pulse-modulation-based power amplifiers. Several examples of pulse width modulation techniques are detailed in “A Review and Comparison of Pulse Width Modulation (PWM) Methods For Analog and Digital Input Switching Power Amplifiers” by Karsten Nielsen, which is incorporated herein by reference in its entirety. Further discussion of PWM can be found in “Comparing Nonlinear With Linear Control Methods for Error Correction in Switching Audio Amplifier Output Stages,” by Thomas Taul, Karsten Nielsen, and Michael A. E. Andersen, which is hereby incorporated by reference. In one embodiment, the pulse width modulated digital amplifier may be a Class D amplifier.